Question

What will be the resultant pH when 200mL of an aqueous solution of HCl (pH = 2.0) is mixed with 300mL of an aqueous solution of NaOH (pH = 12.0) ?​

• A. 3.5
• B. 2.69
• C. 11.3
• D. 8.8

Answer 1

The correct option is C

11.3

pH of HCl solution = 2

[$H^{+}$] = $1\times {10}^{-2}$ M

Number of m mole of $H^{+}$ in 200mL = $200\times 1\times {10}^{-2}$ = 2

pH of NaOH solution 12.0

$\left[O{H}^{-}\right]$ = $1\times {10}^{-2}$M

Number of m mol of $O{H}^{-}$ in 300mL = $300\times 1\times {10}^{-2}$ = 3

After mixing total volume = 500mL

No. of m mol $O{H}^{-}$ left = 3 – 2 = 1

Molarity of [$O{H}^{-}$] = $\frac{1}{500}$ = $0.2\times {10}^{-2}$ = $2\times {10}^{-3}$

$\therefore$ pOH = 2.6990

$\therefore$ pH = 11.3010